1. Field of the Invention
The present invention relates to a process for the separation of volatile chlorine compounds from exhaust gases resulting from the chlorination of titanium-containing raw materials by treating the exhaust gases with aqueous media in several stages and absorbing the volatile chlorine compounds in said aqueous media.
2. Description of the Prior Art
As a consequence of the increasing severity of regulations concerning the purity of air and water and in view of the necessity to operate chemical processes as economically as possible, efforts are being made to reduce the volume of industrial wastes and to recycle any reusable waste material as a secondary raw material into the production.
In the manufacture of titanium dioxide, the process of the oxidation of titanium tetrachloride in the vapor phase ("chloride process") wherein only solid and gaseous reaction products are generated, is increasingly gaining in importance in relation to processes wherein the titanium-containing raw materials are broken down with acids whereby significant amounts of waste acids are produced, the storage and elimination thereof are difficult and expensive. Titanium tetrachloride is therefore an important initial raw material for the manufacture of titanium dioxide.
Titanium tetrachloride is produced by the chlorination of titanium-containing raw materials in the presence of a carbon-containing compound. Suitable titanium-containing raw materials are natural and synthetic rutile, titanium-containing slags or titanium-containing ores, for example ilmenite. Appropriate carbon-containing compounds are anthracite and coke, for example petroleum coke.
The raw materials are chlorinated in a finely dispersed form at elevated temperatures. In the process, the metal oxides contained in the raw materials are converted substantially completely into the corresponding metal chlorides. Following the precipitation of low volatility metal chlorides from the exhaust gases of the chlorination, the highly volatile titanium tetrachloride is condensed by cooling in a condensation stage to approximately -20.degree. C.
Titanium-containing raw materials usually contain silica compounds. Ilmenite, for example, contains on the average up to 3% by weight SiO.sub.2. In this case, the exhaust gas additionally contains a small amount of silicon tetrachloride.
The exhaust gases are entirely free of water. However, they contain a hydrochloric acid component formed by the reaction of chlorine with water contained, for example, in the initial raw material as natural humidity.
A typical composition of the exhaust gases upon leaving the condensation stage is shown in Table 1. (All gas volume data given hereafter are with reference to standard temperature and pressure).
TABLE 1 ______________________________________ Component Amount (Volume %) ______________________________________ HCl 6-9 TiCl.sub.4 0.10-0.20 SiCl.sub.4 0.01-0.20 N.sub.2 + CO.sub.2 + CO balance ______________________________________
Upon contact with water, titanium chloride is hydrolytically cleaved. When the TiCl.sub.4 vapor is introduced into water, initially molecularly dispersed titanium acids are formed which "age" rapidly and pass into a dispersed colloid of titanium dioxide hydrate. This hydrolysis occurs readily upon contact of TiCl.sub.4 vapor with atmospheric humidity whereby an intensely white colored smoke is generated consisting of fine, stable particles of the order of magnitude of micrometers. Such particles are absorbed during conventional washing processes with aqueous media only with considerable difficulty and even pass through multistage washing installations without alteration. Silicon tetrachloride also hydrolyzes upon contact with atmospheric humidity with the formation of smoke, but the optical density of the latter is less than that of the smoke generated during the hydrolysis of TiCl.sub.4.
Exhaust gases containing TiCl.sub.4 and/or SiCl.sub.4 therefore must not be released into the atmosphere without further purification measures, as this would lead to the contamination of the environment.
A process for the treatment of a flow of mainly inert gases containing appreciable amounts of hydrochloric acid and titanium tetrachloride with a wash liquid on an aqueous basis is described in DE-OS No. 22 36 843. In this method, the flow of gas is treated prior to the washing process by evaporating a certain volume of water into the flow of the anhydrous mixture of gases. This amount of water, in order to safely avoid the formation of fog, must be within definite limits. It must be less than the amount required for the saturation of the flow of gas with water and larger than the volume necessary for the stoichiometric reaction with the TiCl.sub.4. Furthermore, this amount of water must be evaporated into the gaseous flow at least 1/20 second before the washing process itself so that an equilibrium may be established between it and the TiCl.sub.4 before the two components enter into contact with the excess volume of water in the washing process.
In the process according to DE-OS No. 27 02 515, a flow of exhaust gases containing a halide waste product, for example titanium tetrachloride, is contacted with an absorbent fog containing water in which an inorganic compound, such as for example hydrochloric acid, is dissolved while observing certain conditions of temperature, the water vapor pressure of the absorbent aqueous fog and the ratio of the surface dimensions of its particles to the weight of the waste halide to be removed.
Special installations are required for the generation of the fog, for example columns with deflecting surfaces, jet washers, spray towers or atomizers.
According to this process, in the course of the absorption of titanium tetrachloride from the flow of exhaust gas, hydrochloric acid solutions with a slight titanium content are obtained with the titanium content possibly amounting after contact with the exhaust gas according to the process of DE-OS No. 27 02 515 to approximately 2% by weight Ti, with respect to HCl.
Such titanium containing hydrochloric acid solutions are sensitive to hydrolysis and tend to precipitate turbidity of hydrated titanium oxides in case of a change in concentration, temperature and/or pH value, which renders them unsuitable for numerous industrial applications, such as for example the regeneration of ion exchangers. Such titanium-containing hydrochloric acids must therefore be purified prior to any industrial use, for example by distillation, which is expensive. In actual practice, the only acceptable solution is their neutralization and subsequent elimination.